Wavelength flexibility through variable-period poling of a compact cylindrical optical fiber assembly

ABSTRACT

A cylindrical electrode module of a fiber optic laser system includes an inner cylinder having an inner repeating pattern of longitudinally-aligned positive and negative electrodes on an outer surface of the inner cylinder. The cylindrical electrode mode includes an outer cylinder that encloses the inner cylinder. The outer cylinder that has an outer repeating pattern of longitudinally-aligned negative and positive electrodes on an inner surface of the inner cylinder that are in corresponding and complementary, parallel alignment with the positive and negative electrodes of the inner repeating pattern on the outer surface of the inner cylinder. The cylindrical electrode module includes an optical fiber having an input end configured to align with and be optically coupled to a pump laser. The optical fiber is wrapped around the inner cylinder within the outer cylinder to form a cylindrical fiber assembly. The electrodes are activated to achieve quasi-phase matching.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119(e)to U.S. Provisional Application Ser. No. 63/150,614 entitled “High PowerLaser Using Quasi-Phase-Matched Electric Field Induced OpticalParametric Amplification in Hollow Core Photonic Crystal Fibers”, filed18 Feb. 2021.

This application claims the benefit of priority under 35 U.S.C. § 119(e)to U.S. Provisional Application Ser. No. 63/150,698 entitled “Quantumentanglement system using quasi-phase-matched electric field inducedoptical parametric amplification in hollow core photonic crystalfibers”, filed 18 Feb. 2021.

This application is a continuation-in-part under 35 U.S.C. § 120 to U.S.patent application Ser. No. 16/986,408 entitled “Wavelength Flexibilitythrough Variable-Period Poling of a Compact Cylindrical Optical FiberAssembly,” filed 6 Aug. 2020, which in turn claimed the benefit under 35U.S.C. § 120 to U.S. patent application Seri. No. 16/920,994 entitled“Wavelength Flexibility Through Variable-Period Poling of Fluid-FilledHollow-Core Photonic Crystal Fiber,” filed 6 Jul. 2020, which in turnclaimed the benefit of priority under 35 U.S.C. § 119(e) to U.S.Provisional Application Serial No. 62/872,316 entitled “WavelengthFlexibility Through Variable-Period Poling of Fluid-Filled Hollow-CorePhotonic Crystal Fiber,” filed 10 Jul. 2019, the contents of all ofwhich are incorporated herein by reference in their entirety.

ORIGIN OF THE INVENTION

The invention described herein was made by employees of the UnitedStates Government and may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefore.

BACKGROUND 1. Technical Field

The present disclosure generally relates to fiber laser systems, andmore particularly to wavelength adjustable fiber laser systems.

2. Description of the Related Art

Applications exist for laser communication, sensing, and designatingthat benefit from being operable in more than one frequency. However,providing more than one laser each operable in respective frequenciescan be prohibitive in cost, size, weight and, power (CSWAP).Alternatively, it is generally known that wavelength conversion of asingle laser can be achieved with nonlinear crystals by nonlinearoptical processes such as sum frequency and difference frequencygeneration or more specifically with optical parametric generation(OPG), optical parametric amplification (OPA), and optical parametricoscillation (OPO). However, efficient operation of typical processes arebelieved to require housing of the nonlinear crystal in an opticalcavity external to the pump laser, which seem to function near opticaldamage threshold and are opto-mechanically sensitive. Thus, thisalternative is also CSWAP prohibitive. Dynamic wavelength agility can beachieved by pump power modulation above and below the nonlinear processoperational threshold or by using a phase modulator to change the pumppolarization state such that the requisite phase matching conditions areinhibited.

In optical parametric generation (OPG), a high power pump laser isshined through a nonlinear medium with a second order nonlinearity (χ²).OPG is also referred to as difference frequency generation. Opticalparametric amplification (OPA) is similar to OPG. OPA uses a high powerpump that is shined in along with a weak seed laser. The seed can beeither the corresponding signal or idler wavelength. The seed isgenerated while the complimentary signal or idler is generated. If thephase matching condition (photon conservation of momentum) is met andsufficient pump power is used, the OPA process can be used to generatetwo longer wavelength photons called the signal (shorter or desiredwavelength) and idler (longer wavelength). The phase matching conditionis often met by sending the pump laser through a χ² nonlinear crystal ina very specific direction and with a specific polarization. Thistechnique is called birefringence phase matching.

Although this technique enables OPA operation the efficiency issometimes lower than ideal because the conditions for phase matchingusually come at the cost of directing the laser beam through the crystalin a direction for which the nonlinearity is low, the crystals areshort, and the crystals can only be exposed to a limited pump powerbefore damage.

To remedy this problem another technique called quasi-phase matching(QPM) is used to simultaneously achieve phase matching and highnonlinearity. The crystal nonlinearity is modified by applying aperiodic high voltage to the crystal. At high enough voltage, thisprocess produces a permanent periodic inversion of the crystalpolarization. The poling period is chosen to enable phase matching for aspecific signal wavelength. The process is permanent and the crystal canonly be used to produce a specific wavelength. Although an effectivesolution to low nonlinearity, the periodically poled crystals arerelatively short and sensitive to optical damage since they require veryhigh pump intensity. Also, this technique requires the addition of freespace optics to couple the pump laser to the crystal. If reduced pumppower is required then the crystal has to be placed in an opticalcavity.

A fiber based solution is based on gas or liquid filled hollow-corephotonic crystal fiber (HCPCF) and solid core fibers. χ² is typicallynegligible for glass, gases and liquids due to the random molecularorientations; the media is centrosymmetric and lacks a significant chi-2value. However, application of an electric field to a glass, gas, orliquid can induce an effective χ². So if the gas-filled HCPCF issandwiched between two electrodes with a voltage applied to theelectrodes, a χ² will be induced but the phase matching condition willlikely not be met. Pressure tuning the gas along with the fiberproperties enables some level of control over the phase matchingconditions and may enable higher order modes to be phase matched. Thisis unattractive because the higher modes will yield lower conversionefficiency and yield a poor beam quality laser emission.

An alternative is to enable quasi-phase matching by applying a periodicelectric field along the length of the gas filled HCPCF similar toperiodically poled niobate. The field will induce a χ² and proper polingperiod will enable phase matching of the fundamental modes. This wasproposed in 2016 (Broadband electric-field-induced LP01 and LP02.secondharmonic generation in Xe-filled hollow-core PCF) by JEAN-MICHEL MÉNARD,but an enabling method to implement was not contemplated or disclosed.

SUMMARY

The present innovation overcomes the foregoing problems and othershortcomings, drawbacks, and challenges of wavelength adjustable fiberlaser systems by providing a compact, method of quasi-phase matching inoptical fibers. While the present innovation will be described inconnection with certain embodiments, it will be understood that theinvention is not limited to these embodiments. To the contrary, thisinvention includes all alternatives, modifications, and equivalents asmay be included within the spirit and scope of the present invention.

According to one aspect of the present innovation, a cylindricalelectrode module of a fiber optic laser system includes an innercylinder having an inner repeating pattern of longitudinally-alignedpositive and negative electrodes on an outer surface of the innercylinder. The cylindrical electrode mode includes an outer cylinder thatencloses the inner cylinder. The outer cylinder that has an outerrepeating pattern of longitudinally-aligned negative and positiveelectrodes on an inner surface of the inner cylinder that are incorresponding and complementary, parallel alignment with the positiveand negative electrodes of the inner repeating pattern on the outersurface of the inner cylinder. The cylindrical electrode module includesan optical fiber having an input end configured to align with and beoptically coupled to a high power pump laser. The optical fiber iswrapped around the inner cylinder within the outer cylinder to form acylindrical fiber assembly. The electrodes are activated to achievequasi-phase matching. In one or more embodiments, at least one of thepattern of electrodes is fabricated using additive manufacturing.

According to another aspect of the present innovation, a fiber lasersystem includes a pump laser, a cylindrical electrode module, and acontroller. The cylindrical electrode module includes an inner cylinderhaving an inner repeating pattern of longitudinally-aligned positive andnegative electrodes on an outer surface of the inner cylinder. An outercylinder encloses the inner cylinder and that has an outer repeatingpattern of longitudinally-aligned negative and positive electrodes on aninner surface of the inner cylinder that are in corresponding andcomplementary, parallel alignment with the positive and negativeelectrodes of the inner repeating pattern on the outer surface of theinner cylinder. An optical fiber has: (i) an output end; and (ii) aninput end aligned with and optically coupled to the high power pumplaser. The optical fiber is wrapped around the inner cylinder within theouter cylinder to form a cylindrical fiber assembly, the output endextending out of the outer cylinder. The controller is communicativelycoupled to, and activates, the inner repeating pattern of positive andnegative electrodes and the outer repeating pattern of negative andpositive electrodes to perform adjustable period poling to achieve QPMwith wavelength agility in a compact form factor. In one or moreembodiments, at least one of the pattern of electrodes is fabricatedusing additive manufacturing.

Additional objects, advantages, and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The description of the illustrative embodiments can be read inconjunction with the accompanying figures. It will be appreciated thatfor simplicity and clarity of illustration, elements illustrated in thefigures have not necessarily been drawn to scale. For example, thedimensions of some of the elements are exaggerated relative to otherelements. Embodiments incorporating teachings of the present disclosureare shown and described with respect to the figures presented herein, inwhich:

FIG. 1 depicts a fiber laser system that dynamically changes signalwavelength of, according to one or more embodiments;

FIG. 2A is an isometric diagram illustrating a gas or fluid-filledhollow-core photonic crystal fiber (HCPCF) with orthogonally opposed,periodic electrode structure, according to one or more embodiments;

FIG. 2B is an isometric diagram illustrating the gas or fluid-filledHCPCF of FIG. 1 with a second pair of orthogonally opposed, periodicelectrode structure that is orthogonal to the first pair, according toone or more embodiments;

FIG. 3A depicts a diagrammatic side view of a linear fiber assembly witha single pair of electrode sets on diametrically opposed sides of anoptical fiber, according to one or more embodiments;

FIG. 3B depicts a diagrammatic side view of a linear fiber assembly withperiodic groupings of three (3) positive electrodes and periodicgrouping of three (3) negative electrodes, according to one or moreembodiments;

FIG. 3C depicts a cross sectional diagrammatic side view of a linearfiber assembly with four selectable pairs of electrode sets, accordingto one or more embodiments;

FIG. 3D depicts a side diagrammatic side view of a linear fiber assemblywith a single pair of electrode sets that is spiraled around the opticalfiber with complementary corresponding positive and negative electrodebeing diametrically opposed, according to one or more embodiments;

FIG. 4 depicts a three-dimensional, disassembled view of the cylindricalfiber assembly of FIG. 1, according to one or more embodiments;

FIG. 5 is a graphical plot of electrode poling period versus signalwavelength for different xenon (Xe) pressures and HCPCF diameters,according to one or more embodiments;

FIG. 6 is a graphical plot for a liquid-filled HCPCF that uses carbondisulfide (CS₂), according to one or more embodiments;

FIG. 7 is a three-dimensional view of an example multiple-periodcylindrical fiber assembly, according to one or more embodiments;

FIG. 8 is a diagram of multiple electrode patterns with complementarytop-bottom electrode pairs across fiber, according to one or moreembodiments;

FIG. 9 is a three-dimensional view of creating concentric cylindricalshells with fiber sandwiched there between, according to one or moreembodiments;

FIG. 10 is a three dimensional view of fabricating another examplecylindrical electrode, according to one or more embodiments;

FIG. 11 is a three-dimensional view of assembling an example electrodeassembly using additive manufacturing, according to one or moreembodiments;

FIG. 12 is a depiction of an electrode structure with interdigitatedopposite electrode patterns printed in multiple strips per sheet,according to one or more embodiments;

FIG. 13 is a depiction of another electrode structure with mirroredopposite electrode patterns with varied periodicity printed onrespective strips, according to one or more embodiments;

FIG. 14 is a three dimensional view of assembling the electrode of FIG.13, according to one or more embodiments;

FIG. 15 is a depiction of an accordion-shaped electrode structure,according to one or more embodiments:

FIG. 16 is a depiction of a spiral electrode structure, according to oneor more embodiments; and

FIG. 17 is a depiction of the spiral electrode structure assembled witha fiber, according to one or more embodiments.

DETAILED DESCRIPTION

According to aspects of the present invention, wavelength flexibilitythrough variable-period poling, or variable period quasi-phase matching(QPM), of fibers, including liquid or gas-filled hollow-core photoniccrystal fiber (HCPCF) and solid core fibers, enables wavelengthflexibility in a fiber based laser system for applications such ascommunications, three-dimensional laser scanning (“LADAR”), militaryilluminators, beacons, and designators. In particular, applyingdifferent poling periods around the fiber enables rapid wavelengthagility. In one or more embodiments, the fiber based laser system is oneor more of: (i) a simple, small, low cost and efficient; (ii)opto-mechanically insensitive to misalignment; and (iii) exhibit rapidwavelength switching; and (iv) provide considerable flexibility inwavelength selection.

In one or more embodiments, the present innovation uses solid corefibers. In one or more embodiments, the present innovation uses gas orliquid filled fibers. In one or more embodiments, the present innovationuses temporarily gas or liquid filled fibers. In one or moreembodiments, aspects of the present innovation provide periodic polingof a gas/liquid filled hollow core fiber or solid core fiber withdynamically adjustable poling period to enable wavelength flexibility.In contrast to the crystal, the effect of the poling will not bepermanent; once the field is turned off the molecules will relax andthere will be no optical axis. Thus, an optical parametric amplification(OPA) process can be turned on and off by turning the electrode voltageon and off. If the electrode period can be dynamically changed, the OPOprocess can enable the phase matching condition to be met for differentsignal and idler wavelengths.

In one aspect, the present disclosure provides dynamic wavelengthagility. The ability to dynamically change wavelength is achieved byenergizing different electrodes where different electrodes havedifferent periods. Each period is designed to produce a particularwavelength. In one or more embodiments, a second technique uses a singleelectrode poling period with modulating power to the electrodes. So whenthere is no power you get the pump wavelength out and when there is avoltage applied to the electrode you get the design wavelength.

The present innovation has a number of applications that include opticalparametric generation (OPO), optical parametric amplification (OPA), andoptical parametric oscillation (OPO). In OPG, the input is one lightbeam of frequency ω_(p), and the output is two light beams of lowerfrequencies ω_(s) and ω_(i), with the requirement ω_(p)=ω_(s)+ω_(i).These two lower-frequency beams are called the “signal” and “idler”,respectively. This light emission is based on the nonlinear opticalprinciple. The photon of an incident laser pulse (pump) is divided intotwo lower-energy photons. The wavelengths of the signal and the idlerare determined by the phase matching condition. The wavelengths of thesignal and the idler photons can be tuned by changing the phase matchingcondition.

In one aspect, the present innovation provides wavelengthflexibility—reconfigurable electrodes to produce different periods ormultiple period electrodes around the fiber. Generally-known approachesare limited to solid core fibers and use a planar ground electrode andnot a periodic electrode of oppose polarity on either side of the fiber.The generally-known approach only achieved a conversion efficiency ofabout 0.1% and only focused on second harmonic generation. By usingperiodic electrodes of opposite polarity opposed to one another, our OPAis anticipated to have a conversion efficiency of approximately 30%based on modeling. The SHG and OPA process are similar enough that onecan directly compare the efficiencies. Also, we propose using additivemanufacturing or lithography techniques for electrode production toenable very short periods—as short at microns for lithography or ˜10'sof microns for additive manufacturing. The generally-known approachesuse printed circuit board with electrode period of ˜1.5 mm. This shortperiod forces them to work at low pressure where nonlinearity chi-2 islow. Highest chi-2 occurs at highest pressure which requires shortestelectrode period. Although some generally-known approaches use QPM,there was no suggestion of dynamic wavelength flexibility.

FIG. 1 depicts a diagram of a fiber laser system 100 that providesdynamic wavelength agility by configuring paired periodic electrodes 102a-102 b positioned on opposite sides of an optical fiber 104 provide afiber assembly 106 for adjusting quasi-phase matching. In one or moreembodiments, the paired periodic electrodes 102 a-102 b and opticalfiber 104 are arranged in a linear fiber assembly 106 a. In addition,the electrode period at different azimuthal angles along the cylinderare different—the phase matched wavelengths will vary depending on whichelectrode is powered. The poling period necessary to achieve phasematching for a particular signal wavelength is a function of the pumpwavelength, desired signal/idler wavelengths, fiber design and effectivemodal index of refraction. For a gas-filled fiber, pressure can be usedto tune the modal index of refraction. In one or more embodiments, thepaired periodic electrodes 102 a-102 b and optical fiber 104 arearranged in a cylindrical fiber assembly 106 b. In one or moreembodiments, the fiber 104 is 250 μm in diameter and is wrapped within acylindrical electrode 107 is approximately 300 μm and encases the fiber104 to form a cylindrical fiber assembly 106 b.

In one or more embodiments, the fiber laser system 100 is configured asan OPA with a pump output 108 from a high power pump laser 110 beingcombined by a dichroic mirror or fiber combiner (“combiner”) 112 with aseed output 114 from a seed laser 116. The seed output 114 can have asignal wavelength or an idler wavelength. The combiner 112 directs thepump output 108 and the seed output 114 in a single direction forcoupling to the fiber assembly 106 via a first lens 118 at one end of anoptical cavity 120 that contains the fiber assembly 106. In one or moreembodiments, the optical fiber 104 of the fiber assembly 106 has a solidcore. In one or more embodiments, the optical fiber 104 of the fiberassembly 106 is has a hollow core that is filled with a fluid comprisingone or more of a gas and a liquid. In one or more embodiments, thehollow core of the optical fiber 104 is encapsulated between an inputgas cell 124 and an output gas cell 126 that include a windowrespectively to receive combined input 128 to and direct output 130 fromthe fiber assembly 106. A collimation lens 132 collimates the output130. A first output dichroic mirror 134 separates residual pump laser136 from beam path 138. A second output dichroic mirror 140 separatesresidual idler laser 142 from beam path 138. In one or more embodiments,the seed laser 116 receives residual pump laser or 136 residual idlerlaser 142. What remains in beam path 138 is laser output signal beam144. In an exemplary embodiments, the pump laser 110 produces pumpoutput 108 having a wavelength of 1.06 μm. which results in residualidler laser 142 of wavelength 3.1 μm and laser output signal beam 144 ofwavelength 1.6 μm.

In one or more alternative embodiments, the fiber laser system 100includes two mirrors 146 a-146 b that define a mirror cavity 148 tooperate as an optical parametric oscillator (OPO). The OPO employs thesame fundamental nonlinear process, difference frequency generation, asis used for the OPA. The difference is that the nonlinear medium (herethe quasi-phase matched) of fiber assembly 106 is part of the opticalcavity 120 that serves to reduce pump power threshold and increaseconversion efficiency. Mirrors 1146 a-146 b reflect the signalwavelength with high reflectivity so that the signal wavelengthoscillates in the mirror cavity 148. This reduces threshold pump powerand increases efficiency. Neither the pump nor the idler oscillate butleak out instead.

FIG. 2A depicts a linear fiber assembly 206 a that includesgas/liquid-filled HCPCF 204 with periodic electrode structure or set 202a-202 b enabling QPM OPA laser generation. Reconfigurable electrodescould be constructed by making the electrodes much smaller than thenecessary period for phase matching and dynamically grouping electrodesto produce the desired periods. Integrated circuit techniques could beemployed to dynamically group electrodes. The electrode structure couldbe produced through lithography or additive manufacturing techniquesdepending on the required periods.

FIG. 2B depicts an alternative design of a linear fiber assembly 206 a′that includes gas/liquid-filled HCPCF 204 gas/liquid-filled HCPCF 200 bthat employs two orthogonal electrode sets or patterns 202 a-202 b, 204a 204 b. In one or more embodiments, electrode sets or patterns can beused that are at angles that are not orthogonal to each other. Thepoling period of the top and bottom electrode pattern 202 a-202 brespectively has a different poling period than the orthogonal right andleft electrode pattern 204 a-204 b. By switching voltage between top andbottom, electrode pattern 202 a-202 b and right and left electrodepattern 204 a-204 b quasi-phase matching is achieved at two differentsignal wavelengths. In addition, one could rotate this electrodestructure around the fiber such that at different azimuth angles thereare different electrode periods which phase match different wavelengths.

FIG. 3A depicts a diagrammatic side view of a linear fiber assembly 306a with a single pair of electrode sets 302 a-302 b on diametricallyopposed sides of optical fiber 304 and that alternate positive andnegative electrode 307 a, 307 b. Wavelength agility can be achieved byactivating and deactivating all of the positive and negative electrode307 a, 307 b. In one or more embodiments, a periodic subset of thepositive and negative electrode 307 a, 307 b is activated anddeactivated to perform wavelength agility.

FIG. 3B depicts a diagrammatic side view of a linear fiber assembly 306b with a single pair of electrode sets 303 a-303 b on diametricallyopposed sides of optical fiber 304 and that repeatedly alternate in agrouping of three (3) positive electrodes 307 a and a grouping of three(3) negative electrode 307307 b. Wavelength agility can be achieved byactivating and deactivating all of the grouped positive and negativeelectrode 307 a, 307 b. In one or more embodiments, the positive andnegative electrode 307 a, 307 b are dynamically reconfigurable into adifferent number (e.g., 1, 2, 3, 4, etc.) of repeated groupings ofpositive and negative electrode 307 a, 307 b to perform wavelengthagility.

FIG. 3C depicts a cross sectional diagrammatic side view of a linearfiber assembly 306 c with four selectable pairs of electrode sets 311a-311 b, 312 a-312 b, 313 a-313 b, and 314 a-314 b, that alternatepositive and negative electrode 307 a, 307 b. Each electrode set 311a-311 b, 312 a-312 b, 313 a-313 b, and 314 a-314 b are diametricallyopposed to the corresponding other electrode set 311 a-311 b, 312 a-312b, 313 a-313 b, and 314 a-314 b of the pair. Each electrode set 311a-311 b, 312 a-312 b, 313 a-313 b, and 314 a-314 b is at differentradial positions to other electrode set 311 a-311 b, 312 a-312 b, 313a-313 b, and 314 a-314 b. Wavelength agility can be achieved byactivating one of the four pairs of electrode sets 311 a-311 b, 312a-312 b, 313 a-313 b, and 314 a-314 b that has respectively a differentperiod of positive and negative electrode 307 a, 307 b from other pairs.The first pair of electrode sets 311 a-311 b (“A1”, “B1”) has a periodthat is shorter than the second pair of electrode sets 312 a-312 b(“A2”, “B2”). The third pair of electrode sets 313 a-313 b (“A3”, “B3”)has period that is shorter than the first pair of electrode sets 311a-311 b (“A1”, “B1”). The fourth pair of electrode sets 314 a-314 b(“A4”, “B4”) has period that is longer than the first pair of electrodesets 311 a-311 b (“A1”, “B1”) and that is shorter than the second pairof electrode sets 312 a-312 b (“A2”, “B2”).

FIG. 3D depicts a side diagrammatic side view of a linear fiber assembly306 d with a single pair of electrode sets 321 a-321 b that spiralaround the optical fiber 304 with complementary corresponding positiveand negative electrode 307 a, 307 b diametrically opposed. The spiralshape can provide a structural benefit and can enable a higher densityof electrodes per longitudinal length of fiber.

In one or more embodiments, FIG. 4 depicts cylindrical electrode module401 that encases a long length of wrapped optical fiber 404 thatperforms wavelength agility as a cylindrical fiber assembly 406. Thecylindrical electrode module 401 includes an inner cylinder 405 a thatis nested within an outer cylinder 405 b. The optical fiber 404 is runthrough the cylindrical electrode module 401 between the inner and outercylinders 405 a 405 b to form the cylindrical fiber assembly 406. Anouter surface of the inner cylinder 405 a includes an inner periodicelectrode set 402 a having a repeating pattern of positive and negativeelectrodes 407 a 407 b. An inner surface of the outer cylinder 405 bincludes an outer periodic electrode set 402 b having a complementaryrepeating pattern of positive and negative electrodes 407 a 407 b to theinner periodic electrode set 402 a. The inner and outer periodicelectrode set 402 a 402 b that interact with the long length of theoptical fiber 404 to achieve quasi-phase matching.

In one or more embodiments, the optical fiber 404 is gas or liquidfilled and thus has a longer poling periods than a solid filled fiber. Alonger poling period makes the corresponding larger dimensions of theinner and outer periodic electrode sets 402 a-402 b, 404 a 404 b easierto manufacture. For instance Xe at 50 atmospheres has a period ofapproximately 1 mm whereas fused silica (glass) has a period ofapproximately 100 μm.

Liquid could be used instead of gas. FIG. 5 depicts a graphical plot 500of electrode poling period versus signal wavelength for different xenon(Xe) pressures and HCPCF diameters. FIG. 6 depicts a graphical plot 600for a liquid-filled HCPCF that uses carbon disulfide (CS₂).

If gas/liquid nonlinearity is too low then conversion efficiency will below. This could be remedied by placing the periodically poled gas/liquidfilled hollow fiber inside of an optical cavity to create an opticalparametric oscillator (OPO). OPOs are a common technology for wavelengthconversion but are usually constructed from a nonlinear crystal andcavity mirrors.

A significant problem with operation in a material outside of Noblegases (like Xe) is that parasitic processes like Raman scattering canbeat out the OPO/OPA channeling the pump energy into unwanted Ramanoutput wavelengths. A solution to this is to introduce significant lossat the Raman wavelength so that the OPA/OPO process can dominate.

The following four (4) references are included in the priority documentto the present application and are hereby incorporated by reference intheir entirety: (i)https://www.rp-photonics.com/optical_parametric_generators.html; (ii)https://en.wikipedia.org/wiki/Optical_parametric_amplifier; (iii)Quasi-phase matching. David S. Hum*, Martin M. Fejer, C. R. Physique 8(2007), pp. 180-198; and (iv) Phase-matched electric-field-inducedsecond harmonic generation in Xe-filled hollow-core photonic crystalfiber, JEAN-MICHEL MÉNARD AND PHILIP ST. J. RUSSELL, Vol. 40, No.15/Aug. 1, 2015/Optics Letters 3679. 0146-9592/; and (v) JEAN-MICHELMÉNARD FELIX KÖTTIG, AND PHILIP ST. J. RUSSELL, Vol. 41, No. 16/Aug. 15,2016/Optics Letters 3795.

In one or more embodiments, the present disclosure provides wavelengthflexibility through application of multiple periodic electrodestructures. The specific wavelength that is produced depends on whichelectrode is powered. This process is useful for any nonlinear processthat requires phase matching and can be used in gas or liquid filledfibers and fused silica solid core fibers. The latter includes fiberssuch as telecom fibers.

In one or more embodiments, the present disclosure provides acylindrical electrode that enables use quasi-phase matching on any typeof fibers. This can be for the purpose of creating a specific wavelengthor to achieve wavelength agility. The key is that cylindrical electrodeenables long interaction length in a practical electrode structure. Oneaspect of using the term “cylindrical” is associated with producing anumber of different periodic electrodes (different periods) and wrappingthem around the fiber. By choosing which one is powered, the wavelengthproduced can be chosen. The second usage of “cylindrical” toimplementing single electrode period within cylindrical packaging forreduced length that is more compact and practical. In one or moreembodiments, the cylinder electrode could have different periods atdifferent heights to enable use of multiple electrode periods.

FIG. 7 is a three-dimensional view of example multiple-periodcylindrical fiber assembly 700. The process begins as did the concentriccylinder approach. A cylinder will have periodic electrodes printed inthe azimuthal angle around its surface. An optical fiber is wrappedaround the electrode. Now instead of sliding a slightly larger diametercylinder, with periodic electrodes printed on its inner surface, overthe fiber wrapped inner cylinder, we print the outer electrodes directlyon the optical fiber. This has a couple of advantages. First, itminimizes the distance between the inner and outer electrodes increasingfield strength and reducing field fringing. Both of these factorsincrease nonlinearity and therefore conversion efficiency. Secondly, itremoves concern for mechanical tolerances between the cylinders. Sine wedesire the two cylinders to have as close to the same diameters aspossible without mechanical interference, any mechanical tolerancesrequired to accommodate uncertainties in the mechanical stack up willreduce the conversion efficiency. Since we are printing directly on thefiber there is reducing concern over tolerances.

The additive manufacturing printing process in principle is summarizedas: 1) a sticky substance is sprayed on the cylinder prior to wrappingwith fiber; this ensures the fiber is secured once the material is curedand it also removes air from the stack increasing the breakdown voltage.After the fiber is wrapped another material is sprayed/printed in thespaces between the fibers. This serves to remove air and increaseelectrical breakdown but it also helps to smooth out the outer surfaceon which the outer electrodes will be sprayed. A certain level ofsmoothness is probably necessary to ensure the nonlinear optical processoccurs as predicted for QPM. These materials are cured so that theyharden. Lastly, the outer electrodes is sprayed on. As indicated this isa rough sketch of the process and this will be tested/finalized in thecoming months.

Unfortunately, using a single polarity does require longer fiber, whichreduces conversion efficiency due to fiber loss. The advantage is thatit is much easier to print this electrode configuration. For certainconfigurations that may be a reasonable trade. For example, if a fiberhas high nonlinearity and/or low loss then the reduced field modulationcan be made up for using longer fiber. If the fiber has low nonlinearityand high/moderate loss then conversion efficiency will definitelysuffer. This is the case for Xe filled fiber at low to moderately highpressure. We haven't examined other materials but there may be caseswhere this is an effective design.

Initial work is underway as to appropriate applications and details ofimplementation for quantum tech. The same nonlinear processes previouslydiscussed are relevant but the focus tends to be on spontaneousparametric down conversion (SPDC, difference frequency generation) andspontaneous four wave mixing, SFWM). One big difference here is that thepump powers have to be kept low so that there is no amplification ofsignal/idler; ideally we produce one signal/idler photon per pump pulse.Similar to the current work, a phase, matching condition must be met.

Traditionally, SPDC was accomplished in nonlinear crystals to produceentangled (strong correlation between signal and idler properties) orunentangled (weak/no correlation between signal/idler) photon pairs.Although this works, it is difficult to efficiently couple these photonsinto fibers which form the basis of quantum networks. Also, thesecrystals offer little flexibility in emission wavelengths.

Traditional fibers do not have an inherent 2nd order nonlinearity andcannot produce SPDC. Since fibers have a 3rd order nonlinearity they canbe used for SFWM. The practical difference between SPDC and SFWM is thelocation of the signal and idler relative to the pump wavelength. ForSPDC, the signal and idler are both on the long wavelength side of thepump whereas for SFWM the signal on the short wavelength side of thepump and the idler is on the long wavelength side. There has been somework producing photon pairs using traditional fibers and Xe filledhollow fibers.

The selection of entangled vs unentangled photons is accomplishedthrough a quantity called joint spectral amplitude (JSA) , JSA˜Phasematching function*energy conservation function. To produce unentangledphotons, the appropriate JSA is accomplished by certain constraints ongroup velocity of the pump, signal, and idler fields, An example is thatthat group velocity of pump=signal V, or pump V=idler V. So now a phasematching condition and group velocity matching condition is required.There has been one recent paper on SFWM in XE hollow fiber where theyshowed the ability to meet these conditions through a combination offiber design nd pressure tuning.

There are at least two novel aspects of our quasi-phase matchingtechnique: 1) we can accomplish 2nd order nonlinear optics such as SPDCand 2) we can, potentially, accomplish 3rd order processes such as SFWMin the same, single fiber device. We may need a different pump for eachprocess though. For SPDC, the ability to independently meet groupvelocity (GV) and phase matching conditions gives much greater controland flexibility in design. For example, pressure tuning and fiber designcan be used to meet GV where as QPM can meet phase matching conditions.Our preliminary analysis also shows that we can meet both conditionsover a broad spectral range by pressure tuning. This could be importantas it might be valuable to have biphoton source that can work throughoutthe common fiber telecom bands that span 1460 nm-1625 nm; this might bevaluable in quantum networks for communication or quantum computers.

It may be possible that SFWM could occur in the same fiber. SFWM has aspectral distribution of signal/idler that is more favorable for certainunentangled photon applications requiring heralded photons. Here SFWM isused to produce a near IR signal (˜800 nm) and a telecom idler (˜1550nm) and this allows a super-efficient silicon single photon detector tosense the presence of the signal photon and then let the system usingthe photons know, herald, the idler photon is therefore present. Theidler photon would then be used for communication or computation work.Heralding is necessary because these sources are probabilistic and,given the need to avoid stimulated emission, the source produces ˜0.1photon pairs/pump pulse.

QUASI-PHASE MATCHING OPTICAL FIBERS USING FLEXIBLE PRINTED CIRCUITTECHNOLOGY: We advance the application of flexible printed circuit (FPC)tech. to quasi-phase matched (QPM) nonlinear optical processes inoptical fiber for development of wavelength flexible fiber lasers andsources of quantum states of light. The FPC maybe packaged in acylindrical, linear, or other geometry to enable desired electrodeinteraction length in compact volume. The nonlinear optical processesmay include difference frequency generation, optical parametricamplifiers and oscillators, second harmonic generation, etc. Theapplication of FPC to quasi-phase matching in fiber is new andpotentially disruptive. The disruptive element arises from thepotential, for the first time, to QPM long lengths of optical fiber toenable efficient nonlinear optical processes in any time of fiber at anyhigh transmission wavelength in the fiber. The FPC technology hasrecently been extended to circuits as long as 70 m. Electrical elementsas small as 20 μmm (periods ˜80 μm) can be produced and futuredevelopment may further extend printed lengths and minimal sizeelements.

The traditional methods of quasi-phase matching (QPM) in fiber are basedon external application of electrostatic fields (we also use static Efields) and thermal poling. Application of external fields has beeninvestigated by many groups on gas filed hollow fiber and traditionalfused silica solid core fiber. The research has demonstrated thepotential of QPM in fiber but efficiency and broader adoption of thetechnology has been hampered by the need for long fiber lengths (1-100m) for efficient operation and the difficulty in making long electrodeswith small electrode period (˜10's-100's micron). Thermal poling occursby raising the temperature of an appropriately doped fiber whilesimultaneously applying an external field for some duration. Propertechnique causes a built in periodic electric field so that aftercompletion of the external electrode is no longer needed. Althoughattractive, this technique is limited by weak built in fields, shortfiber lengths, and the decay of the built in fields over time.

Our approach also employs externally applied fields but does so usingflexible printed circuits (FPC). The FPC should enable high efficiencyinteraction along with greatly improved wavelength flexibility. Theefficiency is increased by long interaction length; current FPC can bemade as long as 70 m. Electric field QPM in the literature has limitedefficiency due to electrode lengths limited to ˜10-30 cm. Current FPCtechnology allows for periodic, alternating polarity electrodes withelectrode period >80 um; implementations in literature generally havemuch longer periods which limited wavelength flexibility. thecombination of long FPC with ˜80 um electrode period allows efficientinteraction and considerable wavelength flexibility in gas filled hollowfiber, and a number of different liquid filled hollow fiber and solidfiber geometries. Continued development of FPC tech will yield evensmall electrode periods and longer lengths which will allow even greaterflexibility of fiber geometry and performance.

Another attractive element of the FPC is the way it can be used andpackaged. For instance the FPC technology may be used to construct theinner and outer electrode shells of a cylindrical electrode, which wouldenable an ultra compact laser/quantum source geometry. Although veryattractive this approach has tight mechanical tolerances; we continue towork this approach. A more straight forward implementation is to encasethe fiber in the long FPC and then compactly the package the long FPC

FIG. 8 is a diagram of multiple electrode patterns with complementarytop-bottom electrode pairs across fiber. Create concentric cylindricalshells with fiber sandwiched in between—each shell has complementaryelectrodes that are aligned. We introduce multiple approaches based onadditive manufacturing printing and flexible printed circuits.

I. Two rigid, cylinders: FIG. 9 is a three-dimensional view of creatingconcentric cylindrical shells with fiber sandwiched there between. Usingadditive manufacturing techniques (or possibly lithography), printperiodic electrodes on both the outer surface of the inner cylinder andthe inside surface of the outer (hollow) cylinder. Wrap the fiber aroundthe inner cylinder, and slide the outer cylinder over the wrapped fiber.Region between shells filled with high dielectric strength material.Critical aspects are cylinder diameter size for close fit and rotationalalignment such that opposite polarity electrodes oppose one another.

II. One rigid, inner cylinder: FIG. 10 is a three dimensional view offabricating another example cylindrical electrode. Using additivemanufacturing techniques (or possibly lithography), print periodicelectrodes on the outer surface of the inner cylinder. Wrap the fiberaround the inner cylinder, and fill and smooth the empty space aroundfiber. Directly print the electrodes on the fiber/dielectric fill mediumsurface. This method removes size tolerance concerns of multiplecylinders but introduces additive manufacturing challenges.

III. Flexible shell approach (additive manufacturing): FIG. 11 is athree-dimensional view of assembling an example electrode assembly usingadditive manufacturing. Using additive manufacturing techniques, printperiodic electrodes on two flexible rectangular sheets, which can bewrapped and joined at a seam into a cylindrical shape. Wrap the firstsheet on an inner cylinder, followed by the fiber and the secondelectrode sheet. This method enables initial planar electrodecharacterization and removes size tolerance concerns of multiplecylinders but requires fine rotational alignment.

IV. Flexible shell approach (flexible printed circuits): FIG. 12 is adepiction of an electrode structure with interdigitated oppositeelectrode patterns printed in multiple strips per sheet. This approachfollows geometry of previous flexible shell approach #3, with theadvantage of more proven method to create Case 2 electrodes.Interdigitated opposite polarity electrode patterns are printed inmultiple strips per sheet. These strips are separated; inner sheet iswrapped and secured on cylinder, followed by fiber and out sheetwrapping. Top bus strip is separated from bottom bus strip at aseparation line. Each bus strip includes a positive electrode arrayinterdigitated with a negative electrode array. In one embodiment, thestrips are 22.5 inches long.

FIG. 13 is a depiction of another electrode structure with mirroredopposite electrode patterns with varied periodicity printed onrespective strips.

FIG. 14 is a three dimensional view of assembling the electrode of FIG.13, according to one or more embodiments;

Advanced FPC technology can now enable printed circuits as long as 70 m;this is a recent world record set by TrackwiseDesigns. Incidentally, ourQPM optical parametric amplifier in Xe filled fiber requires a fiberlength of ˜70 m. May also be possible to stitch multiple FPCs togetherto make longer length. Electrode widths as small as ˜20 um can beproduced which enable electrode periods of ˜80 μm. These short periodswould be necessary for many applications using solid core fiber andliquid filled hollow fiber. These type fibers would likely require ˜10 mof fiber for single pass optical amplifiers and generators. FPCtechnology also applicable for optical parametric oscillators whichwould require electrode length of ˜cm's.

FIG. 15 is a depiction of an accordion-shaped electrode structure.Although 10s of meters in length, the FPC technology would allow theelectrode-fiber structure to be shaped to fill a small volume thusmaking the technology practical. Long electrode could be wrapped on acylinder or spiral structure etc., including an accordion shape thatcould be compressed.

FIG. 16 is a depiction of a spiral electrode structure. FIG. 17 is adepiction of the spiral electrode structure assembled with a fiber.Spiral design has advantages of cylindrical electrode but are in a 2-Dplane making fabrication easier. Could be produced through: (i) flexibleprinted circuits; (ii) Electroplated flat substrate followed by laserremoval of regions intended to be insulating between periodicelectrodes; (iii) Could be produced through lithographic techniques.Spiral electrodes fabricated in the plane of the flexible electrode orother substrate with fiber wrapped on the spiral electrode structure.Following fiber wrap, either a second spiral electrode is aligned on topof the fiber or a common voltage plane is place on top. The common Vplane is simply a planar metallic surface.

FIG> While the disclosure has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the disclosure. Inaddition, many modifications may be made to adapt a particular system,device or component thereof to the teachings of the disclosure withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the disclosure not be limited to the particular embodimentsdisclosed for carrying out this disclosure, but that the disclosure willinclude all embodiments falling within the scope of the appended claims.Moreover, the use of the terms first, second, etc. do not denote anyorder or importance, but rather the terms first, second, etc. are usedto distinguish one element from another.

In the preceding detailed description of exemplary embodiments of thedisclosure, specific exemplary embodiments in which the disclosure maybe practiced are described in sufficient detail to enable those skilledin the art to practice the disclosed embodiments. For example, specificdetails such as specific method orders, structures, elements, andconnections have been presented herein. However, it is to be understoodthat the specific details presented need not be utilized to practiceembodiments of the present disclosure. It is also to be understood thatother embodiments may be utilized and that logical, architectural,programmatic, mechanical, electrical and other changes may be madewithout departing from general scope of the disclosure. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present disclosure is defined by the appendedclaims and equivalents thereof.

References within the specification to “one embodiment,” “anembodiment,” “embodiments”, or “one or more embodiments” are intended toindicate that a particular feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present disclosure. The appearance of such phrases invarious places within the specification are not necessarily allreferring to the same embodiment, nor are separate or alternativeembodiments mutually exclusive of other embodiments. Further, variousfeatures are described which may be exhibited by some embodiments andnot by others. Similarly, various requirements are described which maybe requirements for some embodiments but not other embodiments.

It is understood that the use of specific component, device and/orparameter names and/or corresponding acronyms thereof, such as those ofthe executing utility, logic, and/or firmware described herein, are forexample only and not meant to imply any limitations on the describedembodiments. The embodiments may thus be described with differentnomenclature and/or terminology utilized to describe the components,devices, parameters, methods and/or functions herein, withoutlimitation. References to any specific protocol or proprietary name indescribing one or more elements, features or concepts of the embodimentsare provided solely as examples of one implementation, and suchreferences do not limit the extension of the claimed embodiments toembodiments in which different element, feature. protocol, or conceptnames are utilized. Thus, each term utilized herein is to be given itsbroadest interpretation given the context in which that terms isutilized.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The description of the present disclosure has been presented forpurposes of illustration and description, but is not intended to beexhaustive or limited to the disclosure in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope of the disclosure. Thedescribed embodiments were chosen and described in order to best explainthe principles of the disclosure and the practical application, and toenable others of ordinary skill in the art to understand the disclosurefor various embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A cylindrical electrode module for a fiber opticlaser system, the cylindrical electrode module comprising: an innercylinder comprising an inner additively printed substrate having aninner repeating pattern of longitudinally-aligned positive and negativeelectrodes on an outer surface of the inner cylinder; an outer cylinderthat encloses the inner cylinder and comprises an outer additivelyprinted substrate having an outer repeating pattern oflongitudinally-aligned negative and positive electrodes on an innersurface of the inner cylinder that are in corresponding andcomplementary, parallel alignment with the positive and negativeelectrodes of the inner repeating pattern on the outer surface of theinner cylinder; an optical fiber having: (i) an output end; and (ii) aninput end configured to align with and be optically coupled to a highpower pump laser, the optical fiber wrapped around the inner cylinderwithin the outer cylinder to form a cylindrical fiber assembly, theoutput end extending out of the outer cylinder, wherein the electrodesare activated to achieve quasi-phase matching; and dielectric materialthat fills spaces between the optical fiber to present a smooth outercircumference to the outer additively printed substrate.
 2. A fiberlaser system comprising: cylindrical electrode module comprising: aninner cylinder comprising an inner additively printed substrate havingan inner repeating pattern of longitudinally-aligned positive andnegative electrodes on an outer surface of the inner cylinder; an outercylinder that encloses the inner cylinder and comprises an outeradditively printed substrate having an outer repeating pattern oflongitudinally-aligned negative and positive electrodes on an innersurface of the inner cylinder that are in corresponding andcomplementary, parallel alignment with the positive and negativeelectrodes of the inner repeating pattern on the outer surface of theinner cylinder; an optical fiber having: (i) an output end; and (ii) aninput end configured to align with and be optically coupled to a highpower pump laser, the optical fiber wrapped around the inner cylinderwithin the outer cylinder to form a cylindrical fiber assembly, theoutput end extending out of the outer cylinder, wherein the electrodesare activated to achieve quasi-phase matching; and dielectric materialthat fills spaces between the optical fiber to present a smooth outercircumference to the outer additively printed substrate; a pump lasercommunicatively coupled to the optical fiber; and a controllercommunicatively coupled to, and activates, the inner repeating patternof positive and negative electrodes and the outer repeating pattern ofnegative and positive electrodes to perform adjustable period poling toachieve quasi-phase matching (QPM) with wavelength agility.
 3. The fiberlaser system of claim 2, further comprising: a seed laser that emits oneof a seed laser beam at one of a signal wavelength and an idlerwavelength; and an optical combiner that combines the output from thehigh power pump laser and the seed laser, wherein the fiber laser systemis configured as an optical parametric amplifier (OPA).
 4. The fiberlaser system of claim 2, further comprising two opposing mirrorspositioned on opposite axial sides of the optical fiber to form anoptical cavity, wherein the fiber laser system is configured as anoptical parametric oscillator (OPO).
 5. The fiber laser system of claim2, wherein the optical fiber comprise a nonlinear optical crystal thatgenerates an output containing a signal and an idler, wherein the fiberlaser system is configured as an optical parametric generator (OPG). 6.The fiber laser system of claim 2, wherein the controller activates: (i)a periodic first subset of the inner repeating pattern of positive andnegative electrodes; and (i) a corresponding periodic second subset ofthe outer repeating pattern of negative and positive electrodes todynamically adjust the period poling.
 7. The fiber laser system of claim2, wherein the optical fiber comprises a fused silica solid core fiber.8. The fiber laser system of claim 2, wherein the optical fibercomprises a hollow-core photonic crystal fiber (HCPCF) that is filledwith a fluid.
 9. The fiber laser system of claim 8, wherein the fluidcomprises a gas.
 10. The fiber laser system of claim 9, furthercomprising an optical attenuator that attenuates at a Raman wavelengthto attenuate Raman scattering parasitic process.
 11. The fiber lasersystem of claim 9, wherein the gas comprises a noble gas.
 12. The fiberlaser system of claim 11, wherein the noble gas is xenon (Xe).
 13. Thefiber laser system of claim 8, wherein the fluid comprises a liquid. 14.The fiber laser system of claim 2, wherein the controller performsdynamically adjustable period poling of the first pair of periodicelectrode structures to achieve QPM with wavelength agility: in responseto a first mode selection, activates the inner repeating pattern ofpositive and negative electrodes and the outer repeating pattern ofnegative and positive electrodes to perform dynamically adjustableperiod poling to achieve a first wavelength of output that is one ofsignal and idler wavelength; and in response to a second mode selection,deactivates the inner repeating pattern of positive and negativeelectrodes and the outer repeating pattern of negative and positiveelectrodes to achieve a pump wavelength of output.